Method and apparatus for the location of remote conductors by analysis of signals induced in an antenna array

ABSTRACT

An antenna array has three horizontal coils (A,B,C) with equal spacings s, and two coplanar vertical coils (D,E) with a horizontal spacing 2f. When it is located over one or more current-bearing buried conductors, signal currents (A,B,C,D,E) are induced in the coils. The difference value D-E is measured and compared with a value &#34;D-E&#34; calculated from the equations: 
     
         &#34;(D-E)&#34;=2f/(&#34;d&#34;.sup.2 +f.sup.2) 
    
     
         &#34;d&#34;=2s(B-C)/ A-B)-(B-C)!. 
    
     Identity indicates that the signal source is a single conductor. A larger calculated value indicates plural sources with mainly vertical separation. A small calculated value indicates plural sources with mainly horizontal separation.

BACKGROUND OF THE INVENTION

The present invention relates to apparatus and methods for the locationof remote (typically buried) conductors (typically cables or pipelines).Such conductors typically carry complex currents, with many frequencycomponents, owing to induction by various sources such astelecommunications cables, power cables, electrical plant andbroadcasting stations. The present invention is particularly concernedwith the situation where there are several conductors close together.Conventional techniques are unable to deal with such situations, andtend to give unreliable results. My own earlier applicationWO-A-94/19708 does provide an effective technique. By investigatinginduced signals carried by the conductors at a multiplicity of differentfrequencies it is possible to identify a frequency carried exclusivelyby a single conductor and use it to locate that conductor. E.g. anantenna array including three vertically spaced horizontal coils (A,B,C)is located over an apparent conductor position using vertical coils(D,E). For numerous frequencies, the terms SB/(A-B), 2SC/(A-C) and2S(B-C)/ (A-B)-(B-C)! are determined (where S=coil spacing; X=signalstrength of coil X). For an exclusive frequency the three values areidentical and equal to the depth of the conductor.

SUMMARY OF THE INVENTION

The present invention concerns a simple technique for determining thespacing of two conductors. It can be used to determine whether aparticular signal source is in fact a single conductor or a spaced pairof conductors. This may be used in conjunction with another locationtechnique, preferably one according to WO-A-94/19708. If it is confirmedthat a signal originates with a single conductor, then data relating tothat conductor can be reliably determined.

One type of embodiment uses a "3H2V" coil array. That is, there arethree substantially horizontal and two substantially vertical elongatecoils. The "horizontal" coils are mutually parallel and verticallyspaced (most conveniently being one above the other, with their axes ina common vertical plane). The "vertical" coils are mutually parallel andspaced apart. They extend vertically, with their axes preferably in, orclose to, a common vertical plane. This may be the same plane as theplane of the horizontal coils, or it can be a plane at an angle theretoprovided it is not at a right angle thereto. For such an array, it ispossible to calculate the arithmetical difference between the signalsinduced in the two vertical coils by a conductor located symmetricallybeneath them. The calculation involves known or measurable quantities.The actual difference value can also be determined. If the signal sourceis a single conductor, then the calculated value will be identical tothe actual value. If they are found to differ, this indicates that thesignal source is not a single conductor. Depending on what is requiredof the investigation, one would either then calculate the spacing of thesignal sources, or one would change the investigation in an attempt tofind a single source. (Typically this would mean investigating adifferent frequency, generally as discussed in WO94/19708).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a 3H2V aerial coil array being employed ina method of the present invention.

FIG. 2 is a graph for explaining how the spacing of remote conductorsmay be determined by means of the invention.

FIGS. 3 A, B and C are diagrammatic representations of examples ofapplication of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an aerial array having five coplanar coils A,B,C,D,E. Thatis, they are linear elongate coils and their axes fall in the sameplane. This plane is substantially vertical in use. Three of the coilsA, B, C are essentially identical. They extend horizontally, one abovethe other, with a common spacing s. The other two coils D,E areessentially mutually identical. They are mounted vertically, at the samevertical level, at the sides of the lower part of the 3H array. Amounting frame or housing for the coils is indicated schematically at100. This may also house the electronics and controls. As shownschematically in FIG. 1, they may include a respective amplifier/buffer102 for each coil. The outputs are fed to comparators 104,106 whoseoutputs are fed to a microprocessor 108.

A conductor 10 carrying an AC signal will induce voltages in the coilsA,B,C,D,E. The voltage E induced in a coil X depends on the spacing andthe angular relationship.

The five outputs may be treated as two sets of three: the outputs of thehorizontal coils A, B, C, and the outputs of the bottom horizontal coiland the adjacent vertical coils A, D, E. The responses of these sets maybe compared.

The depth d to a conductor that is a source of a "unique" frequency andis directly below the array is calculated as follows:

    "d"=2s(B-C)/ (A-B)-(B-C)!                                  (Equation 1)

(Note: values in quotation marks (as "d") are values calculated fromequations, as opposed to actual values, which will sometimes differ.)

The use of difference terms, (B-C) etc, gives common-mode rejection ofall distant "incident" fluxes, so that only local fluxes come into thecalculations.

(D+E)=0 in this situation, but (D-E) will give a finite value, since theoutputs of D and E are in opposed phase.

The relationship between A, D, and E is sensitive to whether the sourceof the flux originates from one or several, spatially displaced,conductors. The value of (D-E) for a single source at usual depths maybe accurately computed as follows:

    "(D-E)"=2f "A"/√("d".sup.2 +f.sup.2)                (Equation 2)

where "d"="depth" and f=half the D, E horizontal separation.

The "absolute" value of A may be calculated from the "difference"voltages of A, B; B, C as follows:

    "A"=1/2s ((A-B)/(B-C))-1!                                  (Equation 3)

where s is the vertical separation between A and B, and between B and C.

Where a single source is below, (D-E), as measured, will equal "(D-E)"as derived from equations 2 and 3 above. Where several sources exist(D-E)≠"(D-E)". Also A will not equal "A".

The difference in outputs between the vertical coils, (D-E), can bemeasured. It can also be calculated:

    "(D-E)"=2f{(1/2s) ((A-B)/(B-C))-1!}/√"d".sup.2 +f.sup.2)

where "d"="depth" (or distance) to conductor as calculated from equation1.

f=half the horizontal separation of coils D,E s=vertical separation ofadjacent horizontal coils A,B,C.

When the signals derive from a single conductor, then the calculateddifference "(D-E)" is identical to the measured difference (R-E). But ifthere are two spaced conductors responsible for the signals, then thevalues diverge, to an extent which increases with the conductor spacing.FIG. 2 is a graph showing how the conductor separation alters the ratioof the measured and calculated differences, for conductors at a depth of1.0 m.

FIGS. 3A, B and C show a 3H2V aerial array in which the vertical coilspacing s=0.25 m and the horizontal coil spacing 2f=0.2 m. The array isshown as located 1 m above: a single conductor (FIG. 3A); a pair ofconductors horizontally separated by 0.5 m, located symmetricallybeneath the array (FIG. 3B); and a pair of conductors vertically spacedby 0.5 m, located centrally beneath the array (FIG. 3C). The data are asfollows:

FIG. 3A

A=1.0 B=0.8 C=0.667 D=-0.099 E=+0.099

(A-B)=0.2 (B-C)=0.133 (D-E)=0.198

2s(B-C)/ ((A-B)-(B-C)!=-0.99

{(1/2s) ((A-B)/(B-C))-1!}=1.0

"d"=0.99 "A"=1.0 "(D-E)"=0.199 "(D-E)"/(D-E)=1.01

FIG. 3B

A=0.94 B=0.769 C=0.65 D=-0.165 E=+0.165

(A-B)=0.171 (B-C)=0.119 (D-E)=0.33

2s(B-C)/ (A-B)-(B-C)!=1.14

{(1/2s) ((A-B)/(B-C))-1!}=0.87

"d"=1.14 "A"=0.87 "(D-E)"=0.152

"(D-E)"/(D-E)=0.461

FIG. 3C

A₁ =2.0 (A₁ +A₂)/2=1.5 B₁ =1.33 (B₁ +B₂)/2=1.065

C₁ =0.909 (C₁ +C₂)/2=0.834 D₁ 0.384

(D₁ +D₂)/2=-0.242 E₁ =0.384 (E₁ +E₂)/2 =0.242

2s(B-C)/ (A-B)-(B-C)!=0.57

{(1/2S) ((A-B)/(B-C))-1!}=1.77

"d"=0.57 "A"=1.77 "(D-E)"=0.61 "(D-E)"/(D-E)=1.27

The technique may be used to calculate the spacing of conductors.Alternatively it may be simply applied to give a yes/no answer to thequestion: is the array over a single conductor? (More generally, themethod will involve the investigation of a single frequency (or narrowfrequency band) at a time. The question then becomes: does the frequencynow being considered originate with a single conductor?)

Thus the following programme may be carried out automatically.

For each frequency examined:

1. Calculate

    "d": =2s(B-C)/ (A-B)-(B-C)!

2. Measure: (D-E)

3. Determine (to, say, ±5%):

    (D-E)=/≠2f{(1/2s) ((A-B)/(B-C))-1!}/√"d".sup.2 +f.sup.2

4. YES/NO

An alternative approach to deriving "D-E)" has been found to giveconsistently good results, particularly with shallow conductors, forwhich the original approach was less reliable. Instead of equation (2),the following empirical equation (4) is used: ##EQU1##

The value of "d" is taken from equation 1. The ratio (X) of thecalculated value Z to the measured value of (D-E) is indicative of thelocation of the flux source:

    Z/(D-E)=X

For any frequency or band of frequencies, if the flux source isexclusively directly below the antenna, then

    X=1

If there is more than one flux source, with mainly horizontalseparation, then

    X<1

If there is more than one flux source, with mainly vertical separation,then

    X>1

I claim:
 1. A method for investigating buried conductors which act aselectromagnetic signal sources by virtue of currents flowing in them,said method comprising:providing an antenna assembly which comprisesthree vertically spaced coils (A,B,C) whose axes are horizontal and twohorizontally spaced coils (D,E) whose axes are vertical; locating theantenna assembly over a region believed to contain one or more buriedconductors of interest; measuring the signals (D,E) induced in the twovertical-axis coils (D,E) and determining a measured difference value(D-E); measuring the three respective signals (A,B,C) induced in thethree horizontal-axis coils (A,B,C); determining from said threerespective signals and the known geometry of the antenna assembly acalculated difference value "(D-E)" that should be produced if thesignals are induced by a single source; comparing the measured andcalculated difference values thereby to obtain an indication of thesingularity of the source or, as the case may be, an indication of theseparation of plural sources.
 2. A method according to claim 1 whereinthe vertical axis coils (D,E) and the horizontal axis coils (A,B,C) havetheir axes substantially in the same vertical plane; the vertical axiscoils (D,E) are mutually identical and have a spacing of 2f; thehorizontal axis coils are mutually identical and the spacings (A-B, B-C)are identical and equal to s; and the calculated difference value"(D-E)" is calculated by means of the equations:

    "(D-E)"=2f "A"/√("d".sup.2 +f.sup.2)

    "A"=1/2s {(A-B)/(B-C)}-1!

    "d"=2s(B-C)/ (A-B)-(B-C)!

where "A" and "d" represent the values of A and d calculated from theabove equations, d representing the distance of the source from thelowest horizontal-axis coil (A).
 3. A method according to claim 1wherein the vertical axis coils (D,E) and the horizontal axis coils(A,B,C) have their axes substantially in the same vertical plane; thevertical axis coils (D,E) are mutually identical and have a spacing of2f; the horizontal axis coils are mutually identical and the spacings(A-B, B-C) are identical and equal to s; and the calculated differencevalue "(D-E)" is calculated by means of the equation:

    "(D-E)"=2f/("d".sup.2 +f.sup.2)

    "d"=2s(B-C)/ (A-B)-(B-C)!.


4. A method according to claim 1 including a step of locating theantenna assembly vertically over the signal source.
 5. A methodaccording to claim 1 wherein the process of comparing the measured andcalculated difference values is repeated at different frequencies untilan indication of singularity is obtained.
 6. Apparatus for investigatingburied conductors which act as electromagnetic signal sources by virtueof currents flowing in them comprising an antenna assembly whichcomprises three vertically spaced coils (A,B,C) whose axes arehorizontal and two horizontally spaced coils (D,E) whose axes arevertical; comparator means for determining a measured difference value(D,E) between signals induced in the two vertical-axis coils; dataprocessing means coupled to said horizontal axis coils for receivingsignals induced in said horizontal axis coils and arranged to calculate,on the basis of the received signals and the known geometry of saidantenna assembly, a calculated value ("D-E)") of the difference valuebetween the signals induced in the two vertical axis coil that should beproduced if the signals are induced in the two vertical axis coils by asingle source.
 7. Apparatus according to claim 6 wherein the verticalaxis coils (D,E) and the horizontal axis coils (A,B,C) have their axessubstantially in the same vertical plane; the vertical axis coils (D,E)are mutually identical and have a spacing of 2f; the horizontal axiscoils are mutually identical and the spacings (A-B, B-C) are identicaland equal to s; and wherein said data processing means is adapted tocalculate the calculated difference value "(D-E)" by means of theequations:

    "(D-E)"=2f "A"/√("d".sup.2 +f.sup.2)

    "A"=1/2s {(A-B)/(B-C)}-1!

    "d"=2s(B-C)/ (A-B)-(B-C)!

where "A" and "d" represent the values of A and d calculated from theabove equations, d representing the distance of the source from thelowest horizontal-axis coil (A).
 8. Apparatus according to claim 6wherein the vertical axis coils (D,E) and the horizontal axis coils(A,B,C) have their axes substantially in the same vertical plane; thevertical axis coils (D,E) are mutually identical and have a spacing of2f; the horizontal axis coils are mutually identical and the spacings(A-B, B-C) are identical and equal to s; and wherein said dataprocessing means is adapted to calculate the calculated difference value"(D-E)" by means of the equations

    "(D-E)"=2f("d".sup.2 +f.sup.2)

    "d"=2s(B-C)/ (A-B)-(B-C)!.